Cellular communication systems have traditionally used licensed frequency bands, and still use. The 3rd Generation Partnership Project (3GPP) initiative “License Assisted Access” (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed radio spectrum such as the 5 GHz band. The unlicensed spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
In addition to LAA operation, it should be possible to run LTE fully on the unlicensed band without the support from the licensed band. This is called LTE-U Stand Alone.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method needs to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
The LBT procedure leads to uncertainty at the eNodeB (eNB) regarding whether it will be able to transmit a downlink (DL) subframe(s) or not. This leads to a corresponding uncertainty at the user equipment (UE) as to if it actually has a subframe to decode or not. An analogous uncertainty exists in the UL direction where the eNB is uncertain if the UEs actually transmitted or not.
In the unlicensed radio spectrum, eNB must perform listen before talk (LBT) prior to data transmission on unlicensed band. LBT Category 4 with exponential backoff is a non-aggressive scheme that allows good coexistence with Wi-Fi and other unlicensed spectrum users. The discovery signal that is transmitted every 40 ms or so is an important reference signal transmitted to allow the UE to maintain coarse synchronization with the eNB. It will use a more aggressive LBT mechanism to ensure that it is not starved. Even so, due to the load in the band it cannot be guaranteed that it will always succeed. Current assumption is, that the start of the discovery signal is restricted to LTE subframe borders and that the start of regular data transmissions is restricted to a few fixed positions within the subframe, including the subframe border.
In Standalone operations, the PCell will also operate on the unlicensed carrier and thus essential control signals and channels will also be subject to unmanaged interference and LBT. New aspects/challenges if PCell must operate in unlicensed spectrum:                The UE is required to listen to paging requests in the unlicensed spectrum        Mobility must work in an unsynchronized unplanned network        Mobility must work in an environment with dynamic neighbour relations        
Further the carrier (re)selection process (when the network node changes its carrier frequency during operation) becomes more problematic when it is also applied to the PCell (or serving cell in IDLE), because then there is no cell that the UE is “anchored” to during the carrier frequency change.
Paging in legacy LTE occurs in a pre-defined occasion which allows the UE, in idle mode to sleep and only wake-up in time for this occasion. In connected mode, PDCCH decoding is also made efficient by only requiring the UE to monitor for P-RNTI during paging occasion, PO.
Like any other transmission in unlicensed spectrum, paging is subject to LBT, i.e. the eNB may have to postpone the paging transmission to the next PO. Since postponing by an entire DRX cycle is not desirable, a configurable paging occasion window (POW) is provided comprising multiple subframes to support multiple paging opportunities per DRX cycle.
While paging can happen inside or outside the serving cell DMTC it is a good practice by eNB to try to restrict paging within the serving cell DMTC to aid the UE in conserving power. Moreover, since a discovery signal such as the DRS appear in the DMTC it may be desirable to design paging requests to be carried in the DRS subframe. Note that paging request, paging signal, and paging message are used interchangeably.
A problem with receiving the paging request in the DRS is that in some cases, when not transmitted in SF0/5, the UE may need to check two possible SF scrambling codes. In unlicensed spectrum the DRS floats using SF scrambling 0 even when transmitted in subframes 1-4. Similarly, SF scrambling 5 could float into subframes 6-9. Hence the UE may need to perform the two hypotheses for the SF scrambling in order to determine if the SF contains the DRS and potentially the paging request or a normal SF containing user data and paging requests. If the paging request is transmitted in the subframe containing the DRS, the paging request uses the same scrambling code as the DRS instead of the subframe specific scrambling. Similar as system information, scheduling information for the paging message is transmitted on a PDCCH using the common search space, and therefore, it may be transmitted in the same subframe as the DRS.
This can be difficult in the UE since the detecting involves utilizing the full channel bandwidth (up to 20 MHz) if using the first symbol containing the CRS or possibly a narrowband (1.4 MHz) detection later in the subframe using the SSS/PSS signals. The first method can be considered processing intense while the later in increases the memory utilization.
It is therefore a desire to enable efficient paging and DRS reception for the UE.